The Green Bay Packers had a long, successful run with Brett Favre until they decided in 2008 that it was time to give the quarterback job to a rising talent — Aaron Rodgers — even though the future Hall of Famer was still winning games. Now NBC is following the same script in another hugely competitive arena: late-night television.

NBC executives are hoping that their planned transition from Jay Leno to the network’s emerging star, Jimmy Fallon, will bring as much long-term success to “The Tonight Show” as Mr. Rodgers has to the Packers. The move — which includes relocating the show to New York City — is expected to take place by the fall of 2014, even though Mr. Leno has won the ratings race for NBC for two decades, and is still winning.

When his time as host comes, Mr. Fallon will almost surely start out earning considerably less than Mr. Leno because there is much less profit to go around in late-night television. Even as it gets more crowded (names continue to be added, including familiar ones like Arsenio Hall), the business of late-night television is financially challenged and increasingly affected by changing viewing habits.

Mr. Fallon, 38, may someday be declared the ruler of late-night comedy, in the way that Mr. Leno’s predecessor, Johnny Carson, once was, said Robert Morton, once the producer for David Letterman. “But Fallon will be the king of a very small kingdom,” he said.

Longtime producers and executives working in late-night television concede that NBC is taking a risk in planning changes to one of the few remaining areas where the network finishes first. NBC, though, is encouraged by the fact that Mr. Fallon seems to appeal to many of the older viewers who have stuck by Mr. Leno and at the same time is popular with a host of younger, more Internet-focused viewers.

Mr. Fallon “has a real chance to be great,” said one veteran executive in the late-night business, who like others interviewed for this article asked not to identified because of continuing business with different networks and relationships with late-night hosts.

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Jimmy Fallon is expected to take over “The Tonight Show.”Credit
Lloyd Bishop/NBC

Among the viewers who determine financial success in late night, those ages 18 to 49, Mr. Leno still leads the network late-night shows, but with an average rating of just 0.8 (about 996,000 viewers) for the season. Jon Stewart does better on cable, with an average rating of 1.1. But the true leader in late-night ratings is a machine, not a comic. The best rating in the time period from 11 p.m. to 1 a.m., according to Brad Adgate, senior vice president for research at Horizon Media, is “recorded playback on the DVR.”

DVR playback of recorded shows is averaging a 3.1 rating — or almost four million viewers. The late-night comics are threatened by cartoon characters, too: shows in those hours on the Cartoon Network are averaging about a 1 rating, or about 1.3 million viewers. (Mr. Fallon is averaging a 0.5 rating for his NBC show in the 12:35 hour — that’s just 632,000 viewers.)

With diminished numbers come diminished profits. Mr. Leno once made about $150 million a year for NBC. Now the number is probably between $25 million and $40 million, a senior network executive said.

Part of the reason is cost. One executive who has worked on late-night budgets estimated that the bigger network shows like “Tonight” still cost $50 million to $70 million a year. (This is after NBC forced budget cuts this season and won a reduction in salary from Mr. Leno.) That number has to be tightened further for the shows to continue to make money, the executive said.

Another reason is competition. Mr. Leno now faces not only his longtime rival, David Letterman, on CBS, but also Jimmy Kimmel on ABC. Despite initial expectations, though, Mr. Kimmel, 45, has not overrun Mr. Leno, 62, whose resilience (critics call it obstinacy) is something of a television legend. But Mr. Kimmel does edge out Mr. Leno among viewers ages 18 to 34.

Still, youth has always been served in late night. What matters increasingly is how much additional exposure the comedy generated by these hosts enjoys on sites like YouTube and Hulu.

Those areas have been strengths for Mr. Fallon and Mr. Kimmel, both of whom have created widely viewed videos. Mr. Fallon’s dance video with Michelle Obama, for example, has now exceeded 15 million views on YouTube. A version of “Call Me Maybe” with the host and his band, the Roots, accompanying Carly Rae Jepsen on classroom instruments (like kazoos) has surpassed 13 million views. And Mr. Kimmel has churned out hit after hit on video.

Mr. Adgate said an increasing number of younger viewers do not even watch television anymore. He cited a recent Nielsen study that estimated five million households do not have TV sets, adding that “50 percent of those homes have heads of households under 35.”

“That audience hardly knows who Jay Leno is,” he said.

But if “Tonight” is going to sustain its position as a cultural icon, NBC still needs late night’s older, more consistent viewers. “The vast majority of revenue still comes from the television show,” said a veteran late-night producer, who went on to say that even the shows that have significant Internet followings cannot count on much more than $3 million to $5 million a year in revenue from those sources. That will not put a dent in a $50 million-plus budget.

NBC is walking a fine line between retaining the old and embracing the new. One senior executive who has been involved in the planning for Mr. Fallon’s future show suggested that some kind of fresh approach to the format is inevitable, both because the traditional format (monologue, comedy piece, desk, couch, guests, music act, good night) is so old and tired by now and because Mr. Fallon’s talents (impressions, sketch comedy) open up the show to new ideas.

But NBC has looked to reinvent “Tonight” before, and wound up unhappy with the results. The network pushed Mr. Leno out three years ago in favor of the younger, more creatively adventurous Conan O’Brien, then panicked when a horde of mostly older Leno viewers abandoned the show. Mr. Leno was reinstated and NBC lost Mr. O’Brien, a star it had built for 14 years.

NBC’s executives say they are warmed by evidence that Mr. Fallon does not skew too young. They note that the median age for Mr. Fallon’s audience is a surprising 53.3. That is considerably younger than Mr. Leno’s 58.1, but it is not a vast generational difference. Mr. O’Brien dropped the show’s median age precipitously — by a decade — when he hosted “Tonight” in 2009. But he began to lose the contest for total viewers to Mr. Letterman.

One of Johnny Carson’s consistent mantras about success in late night was that a show had to fare well in the middle of the country, where viewers are able to see “Tonight” an hour earlier than on the coasts. Mr. Leno has always demonstrated strength in those areas. Among Mr. Leno’s top 15 cities for viewers age 18 to 49, six are where the show can be seen at 10:35 p.m. rather than 11:35. The number is the same for Mr. Fallon.

“I do think it will be a challenge to keep the Leno ratings,” said a longtime executive who has worked with several of the big late-night hosts. “I always wonder how the younger hosts will handle being in the heat of a presidential election where they have to be accountable and ask tough questions.”

A version of this article appears in print on March 25, 2013, on page B1 of the New York edition with the headline: NBC Risks Its Late-Night Dominance With a ‘Tonight’ Gamble. Order Reprints|Today's Paper|Subscribe